In draft, Wallbreaker will shut down any opponent that hopes to win with the Forge Guardian Alpha and Jet Pack combination (or other similar Set 2 strategy). Moreover, Wallbreaker’s above average health allows it to trade evenly with many commonly played draft commons, which means that Wallbreaker isn’t an awful creature to level up, even in games where its special ability is useless. In short, Tempys players should be able to safely include one or two Wallbreakers in any draft deck—and the “Rare” rarity level means that they will frequently be able to.
In constructed, Wallbreaker Yeti is much more situational—but still potentially powerful in the right circumstances. In particular, Wallbreaker is ideally suited to destroy Brightsteel Gargoyle decks, by instantly killing the Gargoyle (not to mention any buffs currently sitting on the Gargoyle). Not bad for a “comes into play” effect. Of course, this requires that the player playing against the Gargoyle deck be playing Tempys… and I have no idea what the top tier constructed decks will look like post-Set 2, nor if Tempys will be a viable faction. But, if Gargoyle decks remain problematic, Wallbreaker certainly gives top-tier players incentives to splash into Tempys.
Wallbreaker has some interesting interactions with a few other previewed cards. Nuada, Faith’s Flourish is a Legendary Defender with a very strong activated ability. My intuition says that this will be a strong card, and Wallbreaker is ideally suited to shut it down. Steelscale Dragon is another interesting Legendary that will encourage players to experiment with Defender-heavy decks, even though not a Defender itself. I’m not convinced that this will be a particularly strong card—it depends on the other cards in Set 2—but if so, Yeti could be an important part of any counter strategy. Finally, I want to mention Glacial Crush, an anti-Defender spell that was recently spoiled. Glacial Crush is superior removal against Defenders—but is a dead draw if you don’t see any. Wallbreaker Yeti is less potent, but more flexible. This makes Wallbreaker clearly superior in draft; as for constructed, my guess is that Glacial Crush will be slightly preferred as a counter, but I’m not certain.
Finally, there is one last way to play with Wallbreaker Yeti, and that is to build a deck around him. Given Set 1, the most obvious way to do this is to splash Alloyin for Stasis Warden and play some kind of stall deck that uses Defender-inducing spells, Stasis Warden, and Wallbreaker to keep the board clear, plus some late game punch (e.g. Phoenix or Scorchmane in constructed; Forgeplate, Alpha, or Beta in draft) for the finishing power. I think some variant of that deck could work decently well in both constructed and draft, although my instinct is that it is not going to be top-tier in either format. (I like it better in draft, although putting it together is going to be very difficult.) Still, if you like playing around with deck designs, there might be something there.